Background Essay: Migration of the Monarch

The mass migration of monarch butterflies from eastern Canada and the United States to isolated mountain ranges in central Mexico certainly rates as one of the most impressive natural phenomena in the world. Even among birds and mammals, such travel feats are uncommon. Yet, each fall these wispy insects embark on a journey thousands of miles long to a place none of them has ever been before. They fly with only their innate sense of timing and direction to guide them.

Although mass migrations of monarchs occur every year, only every third or fourth generation of monarchs makes the epic flight to Mexico. Between March and September, up to four generations of monarchs may be born, but the majority of these butterflies typically fly only short distances, moving northward during the spring and summer months as they follow the growth of milkweed, their primary food source.

What seems to decide for the monarchs whether they will be local butterflies or far-flung travelers is when during the spring/summer season they are born. First-generation monarchs, the offspring of the generation that spent the winter in Mexico, typically hatch from their eggs in early spring. After spending two weeks as caterpillars and two weeks in the chrysalis stage, they spend their month of adulthood mating, laying eggs, and moving northward. Second-generation and most third-generation monarchs, born in late spring and early summer, live out their two months of life much as their parents before them, mating, laying eggs, and following the milkweed.

Fourth-generation and some third-generation monarchs, however, hatch much later in the season -- July through September -- and are subject to different conditions. These conditions, including shorter days, cooler temperatures, and older and lower-quality milkweed, foreshadow the coming of winter and are probably some of the factors that cue the butterflies that it is time to migrate south.

The journey to Mexico from the eastern United States, experts estimate, takes the average monarch about two months. Like birds, the butterflies probably use a combination of the angle of the sun in the sky and the earth's magnetic fields to guide them southward. It remains a mystery, however, exactly how the monarchs find the relatively tiny patch of land in central Mexico that they will call home throughout the winter. Here the fourth-generation monarchs will stay for the next five or six months -- far longer than the entire life span of their parents. During this time, in a nonreproductive state called diapause, they will not mate or eat. At last, in early spring, the monarchs will come out of diapause, mate, and begin their egg-laying journey northward, where the entire life cycle of this intrepid insect will begin again.